The recruitment of immune cells to sites of injury involves the concerted interactions of a large number of soluble mediators. Several cytokines appear to play key roles in these processes, particularly IL-1 and TNF. Both cytokines are derived from mononuclear cells and macrophages, along with other cell types. Physiologically, they produce many of the same proinflammatory responses, including fever, sleep and anorexia, mobilization and activation of polymorphonuclear leukocytes, induction of cyclooxygenase and lipoxygenase enzymes, increase in adhesion molecule expression, activation of B-cells, T-cells and natural killer cells, and stimulation of production of other cytokines. Other actions include a contribution to the tissue degeneration seen in chronic inflammatory conditions, such as stimulation of fibroblast proliferation, induction of collagenase, etc. They have also been implicated in the process of bone resorption and adipose tissue regulation. Thus, these cytokines play key roles in a large number of pathological conditions, including rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, diabetes, obesity, cancer, sepsis, etc.
The importance of IL-1 in inflammation has been demonstrated by the ability of the highly specific IL-1 receptor antagonist protein (IL-1Ra, or IRAP) to relieve inflammatory conditions (for review, see, e.g., Dinarello (1997) Cytokine Growth Factor Rev. 8:253–265).
IL-1 treatment of cells induces the formation of a complex consisting of the two IL-1 receptor chains, IL-1R1 and IL-1RAcP, and the resulting heterodimer recruits an adaptor molecule designated as MyD88 (Wesche et al. (1999) J. Biol. Chem. 274:19403–19410). MyD88 binds to a protein designated IRAK (IL-1 receptor associated kinase) (see, O'Neill et al. (1998) J. Leukoc. Biol. 63(6):650–657, Auron (1998) Cytokine Growth Factor Rev. 9(3–4):221–237 and O'Neill (2000) Biochem. Soc. Trans. 28(5)557–563, for reviews). IRAK is subsequently phosphorylated and released from the receptor complex to interact with a tumor necrosis factor receptor-associated factor, TRAF6, which transduces the signal to downstream effector molecules (Cao et al. (1996) Nature 383:443–446). TRAF6 can trigger the NIK/IKK kinase cascade to activate the transcription factor NF-κB. NF-κB regulates a number of genes that, in turn, regulate immune and inflammatory responses.
Four IRAKs have been identified: IRAK-1 (Cao, et al. (1996) Science 271:1128–1131), IRAK-2 (Muzio, et al. (1997) Science 278:1612–1615), the monomyeloic cell-specific IRAK-M, also known as IRAK-3 (Wesche, et al. (1999) J. Biol. Chem. 274:19403–10) and IRAK-4 (PCT Publication No. WO 01/051641). IRAK proteins have been shown to play a role in transducing signals other than those originating from IL-1 receptors, including signals triggered by activation of IL-18 receptors (Kanakaraj et al. (1999) J. Exp. Med. 189(7):1129–1138) and LPS receptors (Yang et al. (1999) J. Immunol. 163:639–643; Wesche et al. (1999) J. Biol. Chem. 274:19403–19410). Overexpression of IRAK-2 and IRAK-M has been shown to be capable of reconstituting the response to IL-1 and LPS in an IRAK deficient cell line.
The identification of compounds that modulate the function of IRAK proteins represents an attractive approach to the development of therapeutic agents for the treatment of inflammatory, cell proliferative and immune-related conditions and diseases associated with IRAK-mediated signal transduction, such as rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, diabetes, obesity, allergic disease, psoriasis, asthma, graft rejection, cancer and sepsis.